/etc/asterisk/sip.conf

general]
port=5060
language=de

[21]
type=friend
username=20
secret=20
host=dynamic

[21]
type=friend
username=21
secret=21
language=en

/etc/asterisk/extensions.conf

[sip20]
exten => 20,1,Dial(SIP/20)
exten => 20,n,Hangup

[sip21]
exten => 20.1 Set(Language()=en)
exten => 20,2,Dial(SIP/20)
exten => 20,3,Hangup